Stereoscopic three dimensional (3d) projection systems have been used for many years. One technology known to the art and described, for example, in U.S. Pat. No. 7,528,906B2 dated 23 Jan. 2006 and entitled “Achromatic Polarization Switches”, describes how a polarization modulator can be placed in-front of a single-lens projector, such as a 3-chip DLP digital cinema projector or otherwise.
The projector is arranged so as to generate a single image-beam comprising a rapid succession of alternate left and right-eye images at high speeds of typically 144 Hz (hertz). The polarization modulator imparts an optical polarization state to images generated by said projector and said polarization modulator is operated in synchronization with said projector in order to arrange for all left-eye images to possess a first state of circular polarization and all right-eye images to possess a second state of circular polarization, with said first and second states of circular polarization being mutually orthogonal (i.e possessing opposite senses of rotation, for example with said first optical state comprising clockwise or right-handed circular polarization and said second optical state comprising anticlockwise or left-handed circular polarization).
Thereafter, said left and right-eye images are focused onto the surface of a polarization-preserving projection-screen such as a silver-screen or otherwise, thereby enabling the viewing of time-multiplexed stereoscopic 3d images via utilization of passive circular-polarized viewing-glasses.
Furthermore, it will be known to one skilled-in-the-art that said polarization modulator may comprise of at least one or more liquid crystal elements stacked together in order to achieve the required electro-optical switching characteristics. One technology known to the art for achieving this criterion and described, for example, in U.S. Pat. No. 7,477,206B2 dated 6 Dec. 2005 and entitled “Enhanced ZScreen modulator techniques”, describes how said polarization modulator may comprise of two individual pi-cell liquid crystal elements stacked together in mutually crossed orientation such that the surface alignment-directors in the first pi-cell are orthogonal to the surface alignment-directors in the second pi-cell thereof. Pi-cell liquid crystal elements are known to the art and characterized by the surface alignment-director on each substrate being aligned mutually parallel. Therefore, in at least one optical state the liquid crystal materials composing said pi-cell form a helical structure between said substrates with an overall twist of 180 degrees (i.e pi or π radians). A detailed description of the design and function of pi-cell liquid crystal elements can be found elsewhere in the literature according to the prior-art.
Moreover, each pi-cell liquid crystal element can, for example, be rapidly switched between a first optical state possessing an optical retardation value that is substantially equal to zero when driven with high voltage (eg. 25 volt) in order to switch said liquid crystal materials to the homeotropic texture, and a second optical state possessing an optical retardation value that is substantially equal to 140nm (nanometers) when driven with a low voltage (eg. 3 volt) in order to switch said liquid crystal materials to the splay texture. The homeotropic texture is characterized by the molecular axes of said liquid crystal materials being aligned substantially perpendicular to the surfaces of said substrates, whereas the splay texture is characterized by said molecular axes being aligned substantially parallel with said substrates and furthermore with the twist within said liquid crystal materials being substantially equal to zero. Moreover, said pi-cell liquid crystal elements are capable of being rapidly switched between said first and second optical states thereof at high speeds of greater than typically 250 μs (microseconds) and are therefore often used when designing such polarization modulators according to the state-of-the-art.
It will also be known to one skilled-in-the-art that when said pi-cell liquid crystal element possesses a retardation value substantially equal to 140nm, then said pi-cell constitutes an optical Quarter-Wave-Plate (QWP) for the central part of the visible wavelength spectrum (i.e green wavelengths) and will therefore convert incident linearly polarized visible light to circular polarization.
Therefore, by stacking together two individual pi-cell liquid crystal elements in mutually crossed orientation together with a linear polarization-filter located at the input surface of said stack in order to first convert the initially randomly polarized (i.e unpolarized) incident light generated by said projector to linear polarization, then the images generated by said projector can be rapidly modulated between left and right circular polarization states by operating said pi-cell liquid crystal elements mutually out-of-phase according to the state-of-the-art. Specifically, when said first pi-cell is operated with high voltage (i.e liquid crystal materials are switched to said homeotropic texture) then said second pi-cell is simultaneously operated with low voltage (i.e liquid crystal materials are switched to said splay texture), and vice versa according to the prior-art.
However, since the images generated by a typical 3-chip DLP digital cinema projector are initially randomly polarized (i.e unpolarized), then the linear polarization-filter located at the input surface of said polarization modulator will absorb approximately 50% of the incoming light initially generated by said projector. This will therefore significantly reduce the overall optical light efficiency of said single image-beam system according to the state-of-the-art, thereby resulting in the creation of stereoscopic 3d images that are severely lacking in on-screen image brightness.
One technology known to the art for increasing the overall optical light efficiency of a stereoscopic 3d projection system and described, for example, in U.S. Pat. No. 7,857,455B2 dated 18 Oct. 2006 and entitled “Combining P and S rays for bright stereoscopic projection”, and again in U.S. Pat. No. 8,220,934 dated 29 Sep. 2006 and entitled “Polarization conversion systems for stereoscopic projection”, uses a polarization beam-splitting element in order to split the incoming randomly polarized incident image-beam generated by a single-lens projector into one primary image-beam propagating in the same direction as said original incident image-beam and possessing a first state of linear polarization, and one secondary image-beam propagating in a perpendicular direction relative to said incident image-beam and possessing a second state of linear polarization, with said first and second states of linear polarization being mutually orthogonal.
Thereafter, a reflecting mirror or otherwise is used to modify the optical path for said secondary image-beam and deflect said secondary image-beam towards the surface of a projection-screen, thereby enabling both said primary and secondary image-beams to be arranged so as to mutually overlap to a substantial extent on the surface of said projection-screen thereof. Such double image-beam systems according to the state-of-the-art therefore enable both polarization components composing said initial incident image-beam generated by said projector to be used in order to recreate the overall on-screen image, thereby increasing the resulting image brightness.
Additionally, a polarization rotator is typically required in order to rotate the linear polarization state of said secondary image-beam by substantially 90 degrees and ensure that both said primary and secondary image-beams thereafter possess the same linear state of polarization. Furthermore, one or more polarization modulators are then placed within the optical path of at least one of said primary and secondary image-beams thereof and operated in synchronization with said projector in order to arrange for all left-eye images to possess a first state of circular polarization and all right-eye images to possess a second state of circular polarization, with said first and second states of circular polarization being mutually orthogonal. Stereoscopic 3d images can hence be observed on the surface of said projection-screen via utilization of passive circular-polarized viewing-glasses.
However, the double image-beam system described above according to the state-of-the-art has the disadvantage in that there is a relatively large optical path-length difference between said primary and secondary image-beams thereof, thereby typically requiring the use of a telephoto-lens pair in order to compensate for said optical path-length difference. A telephoto-lens is an optical lens that possesses a relatively long focal-length and which can focus an incident and mutually parallel light-beam to substantially a single point (i.e the focal-point). The telephoto-lens is therefore mandated to have at least one surface that is simultaneously curved around two mutually orthogonal axes in order to create a spherical or ellipsoidal surface, for example with said surface being simultaneously curved around both the horizontal and vertical axes. However, such spherical or ellipsoidal lenses typically suffer from the occurrence of a high level of optical aberration and are also relatively difficult to manufacture which adds both complexity and expense to the overall system.
It will also be understood by one skilled-in-the-art that the aforementioned double image-beam system described above will also be limited in terms of the minimum throw-ratio that can be achieved by said projector. The throw-ratio is defined as being the distance D between the lens of said projector and the surface of said screen, divided by the width W of the image created on said screen (i.e throw-ratio=D/W).
Specifically, when a short throw-ratio is required, the image-beam generated by said projector is mandated to possess a relatively large angle of divergence. Moreover, since the total overall optical path-length for at least the secondary image-beam within said double image-beam system is relatively long, then the high angle of beam divergence will necessitate the requirement of using relatively large optical components such as but not limited to the beam-splitting element, reflecting mirror, polarization rotator and polarization modulators. However, due to limitations on the largest possible sizes of said optical components from both a practical and engineering point-of-view, this limits the maximum value of angular divergence for said image-beam that can be used, thereby also limiting the minimum throw-ratio that said double image-beam system can achieve. Moreover, since many cinema auditoriums often require the use of a projector with a short throw-ratio, this limits the potential usefulness of said double image-beam system according to the state-of-the-art.
An improved system for the displaying of high brightness stereoscopic 3d images according to the state-of-the-art is described, for example, in US patent application publication no. 2015/0103318A1 dated 2 Apr. 2013 and entitled “Stereoscopic image apparatus”, and again in U.S. Pat. No. 9,740,017B2 dated 29 May 2013 and entitled “Optical polarization device for a stereoscopic image projector”. Here, a beam-splitting element is used to separate the randomly polarized incident image-beam generated by a single-lens projector into one primary image-beam propagating in the same direction as said original incident image-beam and possessing a first state of linear polarization, and two secondary image-beams propagating in mutually opposite directions that are also both substantially perpendicular to said original incident image-beam and possessing a second state of linear polarization, with said first and second linear polarization states being mutually orthogonal. The beam-splitting element typically comprises of two plates joined together along one edge to form a chevron or V-shape structure and with the connecting edge for each of said plates being beveled at an angle of substantially 45 degrees in order to allow both said plates to be placed together in close proximity according to the state-of-the-art.
Thereafter, reflecting surfaces such as mirrors or otherwise are used to direct the optical paths for each of said secondary image-beams towards a polarization-preserving projection-screen and arranged such that said primary and secondary image-beams partially overlap in order to mutually combine and recreate a complete image on the surface of said projection-screen thereto. Such triple image-beam systems therefore once again enable both polarization components composing said original incident image-beam generated by said projector to be used in order to generate the overall on-screen image, thereby ensuring for a higher level of image brightness as compared to the previously described single image-beam system thereof.
Additionally, polarization modulators are placed within the optical paths for each of said primary and secondary image-beams and operated so as to modulate the polarization states of said image-beams in synchronization with the images generated by said projector. Specifically, said polarization modulators are typically arranged so as to impart a first circular polarization state to all left-eye images and a second circular polarization state to all right-eye images, with said first and second circular polarization states being mutually orthogonal. Stereoscopic 3d images can therefore be viewed on the surface of said projection-screen via utilization of suitable passive circular-polarized viewing-glasses according to the prior-art.
It will be understood by one skilled-in-the-art that said triple image-beam system described above possesses a relatively small optical path-length difference between said primary and secondary image-beams as compared to the aforementioned double image-beam system thereof, thereby eliminating the necessity of utilizing a telephoto-lens pair in order to compensate for said optical path-length difference. This therefore reduces the overall complexity and cost of the system.
Moreover, it will also be understood by one skilled-in-the-art that since the total overall optical path-lengths for each of said primary and secondary image-beams within said triple image-beam system are relatively small, then said triple image-beam system will be able to operate together with a projector having a shorter throw-ratio as compared to the aforementioned double image-beam system thereof.
However, recently the use of laser projectors in cinema applications has become widely accepted due to their higher level of light output as compared to typical Xenon type cinema projectors. This enables the generation of stereoscopic 3d images with a higher level of on-screen image brightness. However, since the light generated by typical laser projectors is highly coherent and monochromatic, then the central join in the chevron or V-shaped beam-splitting element typically used in said triple image-beam system according to the state-of-the-art often generates a relatively high level of optical diffraction and other related defects, thereby resulting in the perception of on-screen image artifacts near to the middle of said projection-screen. Moreover, the generation of said on-screen artifacts limit the usefulness of said triple image-beam system according to the state-of-the-art when using laser projectors.